The conventional flying spot optical scanner consists generally of a light source, an optical modulator, focusing optics, an optical deflector and a photosensitive surface. The optical modulator, which can be of the acousto-optic type, converts the electronic video data signal supplied to the modulator into a corresponding temporal modulation of the power of the optical beam incident on the modulator. Stated differently, the modulator simply blanks the power of the optical beam on and off. Flying spot optical scanners do not utilize any change in the spatial intensity profile of the light beam. If a light beam having a Gaussian light beam profile enters the modulator, then a Gaussian profile is expected to emerge, albeit possibly subdued in total power, in response to the video signal.
After temporal modulation, the light beam is then hard focussed to a pinpoint and swept across the photosensitive surface by the scanning optics and the optical deflector. For best results, the scanner optics hard focus the modulated light beam to provide a spot size which is infinitesimally small and possessed of rapid risetime.
It has been realized that an optical modulator of the acousto-optic type exhibits a spatial modulation capability. In other words, if a light beam having a Gaussian profile enters an acousto-optic modulator, the modulator truncates this Gaussian profile in a time-varying sequence in response to the video signal. The Scophony scanner, described by D. M. Robinson in Proc. IRE 27,483 (1939), uses an acousto-optic modulator to provide such spatial truncation. The Scophony scanner provides an ability to increase the light beam diameter inside the acousto-optic modulator without suffering resolution degradation. This ability to accommodate larger beam diameters has distinct performance advantages over the flying spot scanner configuration.
A distinctive feature of the Scophony scanner is a broadening of the light profile (the light profile incident on the deflector when the video signal is a pure DC bias with no modulation) observed at the deflector with the introduction of video modulation of the acousto-optic modulator. This broadening is called FM blur. For example, a pure sinusoidal video signal will split the light profile into two video lobes, one corresponding to the positive video frequency component and one corresponding to the negative frequency component. The separation between the centers of these video lobes is directly proportional to the video frequency. The higher the video frequency, the greater will be the broadening of the light distribution. As the video modulation frequency increases, the side-lobes separate until they spill over the boundary of the clear aperture of the deflector. When spillover occurs, the resolution performance of the Scophony scanner degrades. This underscores a fundamental difference between the Scophony scanner and the convential flying spot scanner. In the conventional flying spot scanner no spatial information is intended to be transmitted. If fine spatial structure exists in the scan spot, then the optical system could produce a still smaller scan spot with better resolution. In contrast, the Scophony system exploits the transmission of spatial modulation information to attain peak resolution. Spatial information can be transmitted through the system only when the deflector window (the active reflective surface) is underfilled with the light beam profile. If the deflector window were overfilled, then FM blur could only pull light energy out of the window (by broadening the sidelobes) without introducing any significant spatial modulation inside the window. Spatial information would be lost. On the other hand, if the light profile filled only a fraction of the deflector window, then an appreciable amount of FM blur could be generated without loss of either information or light power. Thus, a direct correlation exists in the Scophony scanner between the spatial extent of the light in the deflector window and the resolution performance.